The present invention relates generally to a shock testing apparatus, and more particularly to a shock testing apparatus designed to simulate pyrotechnic shock (high g short duration) conditions.
Aerospace systems and components often experience pyrotechnic shock conditions as a result of adjacent explosions, for example during separation of stages during flight. As a result of this, components for aerospace systems must normally undergo pyrotechnic shock testing prior to acceptance. There are three basic techniques for pyrotechnic shock testing, all of which have advantages and disadvantages. One technique uses an electrodynamic shaker to produce a desired shock spectrum. However, the amplitude and event duration is limited by the capacity of the shaker. Also, this technique is typically fairly expensive. Another technique employs actual explosives to produce the desired shock. This has the obvious disadvantage of using relatively expensive explosives for each test, and also may have poor repeatability. Finally, mechanical impact techniques may be used to simulate the pyrotechnic shock conditions. In the past, these techniques have also been subject to poor controllability and repeatability.
One known mechanical impact technique for simulating a pyrotechnic shock involves suspending an elongate, metallic beam and exciting it into longitudinal resonance by impact at one end of the beam with a pendulum hammer. Test items are attached to the beam in three orthogonal orientations successively, corresponding to impacts in the x, y and z axis orientations, and the shock response spectrum of the beam is calculated from the measured acceleration using suitably positioned accelerometers. One problem with this is that it does not provide control of the shock response spectrum, or allow any easy means for varying the spectrum to meet alternative test requirements. Normally, different applications require different standard spectra for acceptance testing. Also, peaks are encountered in the response spectrum at the resonant frequencies of the beam. The peaks will normally be outside the allowed tolerance levels of the standard test spectrum, which is generally smooth. This makes it difficult, if not impossible, to generate a wide variety of requested shock spectra.
One previously proposed method of controlling the spectra produced by a resonant bar fixture is to clamp weights at the longitudinal nodal locations of one of the modal frequencies of the bar. The theory predicts that masses clamped at the nodes of the ith mode will cause the response to be dominated by the ith mode.